Pressure die-casting is recognized as a well-established, economical method for manufacturing products and practical for the production of die-cast rotors. Pressure die-casting is widely used in aluminum die-casting. Tool steel mold and accessories used for the aluminum die-casting process have been observed to be inadequate when casting higher melting point metals such as copper. There remains however great interest in building rotors based on copper. Cu-based rotors are more efficient than comparably sized aluminum-based rotors. When the power is kept constant copper-based rotors are smaller than aluminum-based rotors, thereby reducing weight and saving energy. Some high-temperature, high performance materials have been available for many years, e.g., super alloys and refractory metals, such as tungsten and molybdenum. These materials can be used as die materials or a low cost option for use as die insert materials. The primary expense factor for manufacturing die-cast products is the cost of materials. For commercial application the materials noted above cannot be used. The lack of availability of a cheap and durable mold has emerged as an initial technical barrier for commercial manufacturing of copper based die-cast products.
Steel H-13 is a relatively cheap material which allows dry mold release, and fast cycle time. In the United States H-13 steel is denominated as follows: ASTM A681, FED QQ-T-570, SAE J437, SAE J438, SAE J467, UNS T20813. Some unique characteristics of H-13 steel include its ease to work with and availability, making it one of the most popular hot working die steels. Among some of its properties are its thermal shock and fatigue resistance, superior machinability and polishability. In addition, it has proven endurance for mechanical and thermal impact of molten aluminum. However, liquid aluminum is observed to be chemically reactive and easily forms alloys with alloy constituents of H-13 steel dies. This is particularly true when the H-13 steel composition contains copper as one of its constituents.
Researchers have concentrated on developing and evaluating thin coatings on H-13 steel typically applied by physical vapor deposition techniques (PVD), chemical vapor deposition (CVD), and nitriding techniques. Coatings typically deposited by these techniques include CrN, CrC, B4C, VC, CrN2, and ion nitriding. The materials have been chosen due to their resistance to soldering in the presence of liquid Al, which has a melting point of about 660° C. The primary failure mechanism of these coatings is spalling, that is the formation flakes delaminating from the coating, due to differences in the coefficient of thermal expansion (CTE) between the coating and the substrate. Nitridation of H-13 steel prolongs the die life in the presence of liquid aluminum. Even if a coating material is solder resistant, the ability to coat it on a die surface and keep it on a die surface is a technical challenge.
Physical vapor deposition includes heating a source material by resistive heating, plasma sputtering, laser ablation or any other form of energy that will cause the source material to evaporate and “land” on the target material, thus forming a thin layer, coating or thin film. There are several variants for achieving the same goal. In one case source materials may be heated to high vapor pressure by resistive heating. In another variant the source material is heated to high vapor pressure by electron beams under high vacuum. Other heating modes include sputter deposition in which a glow plasma discharge bombards the source material leading to the formation of vapor. Pulsed laser beams may also be used to evaporate the target material. In all cases the vapors condense on the target substrate giving rise to thin coatings or films.
For copper, having a melting temperature of 1083° C., die casting requires pouring temperatures near 1200° C. Casting under these conditions and for copper in particular is characterized by a high heat of fusion, substantial latent heat, and high thermal conductivity. Typical H-13 steel type dies soften at 1200° C. H-13 steel has strength of 100 MPa at about 1200° C., whereas the strength at room temperature is about 1000 MPa. Technical literature shows that some of the major problems associated with H-13 type mold materials at about 1200° C. are: early onset heat checking (thermal strain on surface/appearance of fine cracks due to alternate heating and cooling cycles.), oxidation at high temperature, corrosion and soldering of liquid metal to the stainless H-13 surface, erosion wear by molten metal, and modifying the surface wetting/capillary action after solidification. Furthermore, H-13 steel may in some cases contain up to 0.25% Cu, thereby increasing the possibility of reaction in the liquid solid interface.